My favorite place to get ink refills is Imagination International. They have every color of Copic available and each size including a bunch of accessories. A bottle of ink has about five refills (at least) goes for $9.
So you could spend $35 on five different markers or $14 for one and bottle of ink.
BONUS: They also sell IC screentone! Their variety is huge, the sheets are large (10×14 at least), and the prices are fair! I’ve really been enjoying IC’s stuff.
Shipping is fair and fast, and their website also serves as a remarkable resource for artists who need tips or just want to print out some coloring pages.
Side note: They’re based in Eugene, OR, too, which is home to my alma mater and makes me love them even more! (Go Ducks!)